tire size

I have A 1980 ltd 1000.I have a stock 130/90/16 on the rear.Whats the biggest size tire I can run.With the stock setup?

Width wise, you've already got it. A 130 tire is as big as you should go on that rim. It is possible that you could shoehorn a wider tire on it, but it would create an undesirable tire profile, and could potentially make the contact patch smaller than it already is.